Distributed algorithms for edge dominating sets Jukka Suomela Helsinki Institute for Information Technology HIIT University of Helsinki, Finland PODC, Zurich, 28 July 2010
Edge dominating sets • Simple undirected graph G = ( V , E ) • Edge dominating set D ⊆ E : each edge is in D or adjacent at least one edge in D 2
Edge dominating sets • Any maximal matching is an edge dominating set • x x • But edge dominating sets are not necessarily matchings 3
Edge dominating sets • Any minimum maximal matching is a minimum edge dominating set • Allan & Laskar 1978, Yannakakis & Gavril 1980 • But minimum edge dominating sets are not necessarily matchings 4
Edge dominating sets • NP-hard (and APX-hard) optimisation problem • Simple 2-approximation algorithm : find any maximal matching 5
Edge dominating sets • NP-hard (and APX-hard) optimisation problem • Simple 2-approximation algorithm: find any maximal matching • What about distributed approximation algorithms? • In very weak models of distributed computing • Deterministic algorithms, port-numbering model • Can’t find maximal matchings… 6
Port-numbering model • Identical nodes, 1 no unique identifiers 1 2 3 1 • Port numbers : 2 2 1 • Node of degree d can refer to its neighbours by integers 1, 2, ..., d 1 2 2 • Worst-case analysis: 1 1 1 • Port-numbering chosen 3 2 by adversary 7
Port-numbering model • Focus: • Deterministic distributed algorithms 1 2 2 • Port-numbering model 1 • No restrictions on message size, 1 2 local computation, … 2 1 • Weak model: 1 2 • Can’t break symmetry in cycles • Can’t find graph colouring, maximal matching, … 8
Edge dominating sets in port-numbering model • Problem simple to state: exactly how well can we approximate minimum edge dominating sets • using deterministic distributed algorithms, in the port-numbering model • But why would we care? • Let’s have a look at some classical graph problems from this perspective… 9
Some classical graph problems in port-numbering model Node-based Edge-based vertex cover edge cover Covering Covering problems dominating set edge dominating set Packing independent set matching problems 10
Some classical graph problems in port-numbering model Node-based Edge-based Many packing problems are vertex cover edge cover unsolvable for trivial reasons Covering Covering (impossibility of symmetry breaking in cycles) problems dominating set edge dominating set Packing independent set matching problems 11
Some classical graph problems in port-numbering model Node-based Edge-based Many non-trivial positive results (SPAA 2008, DISC 2008, vertex cover edge cover DISC 2009, SPAA 2010, Covering Covering DISC 2010, …) problems dominating set edge dominating set But trivial lower bounds! Packing (cycles, cliques, etc.) independent set matching problems 12
Some classical graph problems in port-numbering model Node-based Edge-based But do we know vertex cover edge cover anything about Covering Covering edge-based problems covering problems dominating set edge dominating set in this setting? Packing independent set matching problems 13
Edge-based covering problems in port-numbering model • Minimum edge cover seems to be a bit too simple: factor 2 approximation is trivial and tight • But what about minimum edge dominating sets ? • Surprise: both upper bounds and lower bounds are non-trivial! • Contribution: full characterisation of approximability of edge dominating sets in regular graphs and bounded-degree graphs 14
Edge dominating sets: deterministic algorithms in port-numbering model Graph f aph family Approximation ratio d = 1, 3, ... 4 − 6/( d + 1) d -regular d -regular graphs d = 2, 4, ... 4 − 2/ d Δ = 3, 5, ... 4 − 2/( Δ − 1) graphs with graphs with degree ≤ Δ Δ = 2, 4, ... 4 − 2/ Δ Tight results: these are both lower bounds and upper bounds 15
Edge dominating sets: deterministic algorithms in port-numbering model Graph f aph family Approximation ratio Time d = 1, 3, ... 4 − 6/( d + 1) O ( d 2 ) d -regular d -regular graphs d = 2, 4, ... 4 − 2/ d O (1) Δ = 3, 5, ... 4 − 2/( Δ − 1) O ( Δ 2 ) graphs with graphs with degree ≤ Δ Δ = 2, 4, ... 4 − 2/ Δ O ( Δ 2 ) Tight approximation ratios achievable in f ( Δ ) time, f ( n )-time algorithms cannot do any better 16
Lower bound construction: some key ideas • Case: d -regular graphs, d = 2 k • Complete bipartite graph K d , d − 1 • k extra edges (optimal solution) 17
Lower bound construction: some key ideas • Idea: show that there is a port-numbering s.t. any deterministic algorithm has to output a spanning 2-regular subgraph • I.e., a 2-factor (spanning set of disjoint cycles) 18
Lower bound construction: some key ideas • Petersen (1891): any 2 k -regular graph admits a 2-factorisation (partition in 2-factors) = + + 19
Lower bound construction: some key ideas • Use 2-factorisation to assign port numbers : • 1, 2, 1, 2, … in each cycle of 1st factor, 3, 4, 3, 4, … in each cycle of 2nd factor, etc. 1 3 2 2 5 4 6 1 1 2 = + + 20
Lower bound construction: some key ideas • Then we can use covering maps to argue that any algorithm must take all or nothing from each 2-factor 1 3 2 2 5 4 6 1 1 2 5 6 ! + + 4 1 3 2 21
Lower bound construction: some key ideas • Then we can use covering graphs to argue that any algorithm must take all or nothing from each 2-factor • That’s it for even degrees — the case of odd degrees is more difficult • There is always some amount of symmetry-breaking information in port-numbered graphs of odd degree (recall Naor & Stockmeyer 1995) 22
Lower bound: 3-regular 23
Lower bound: 5-regular 24
Algorithm: ≥ 45 (case 1) 25
Algorithm: ≥ 45 (case 2) 26
Optimum: 15 27
Upper bounds: some key ideas • Exploit all possible sources of symmetry-breaking information: • Different node degrees: interpret degrees as colours • Odd degrees: there is a “distinguishable neighbour” • And when symmetry can’t be broken, find a 2-matching (paths and cycles) • On average 1 edge per node • Tricky part: show that this is enough! 28
Upper bounds: some key ideas • Some intuition… algorithm • A really bad case: optimum • 4 edges in algorithm output • 1 edge in optimal solution • What if we had this kind of configuration “everywhere” in a regular graph? • Approximation factor = 4? 29
Upper bounds: some key ideas • This could happen algorithm in an infinite graph but not in a finite graph! optimum • Simple counting argument, different types of endpoints • We can always achieve better than 4-approximation • General case: a bit tedious case analysis, double-counting… 30
Distributed algorithms for edge dominating sets — summary • Small edge dominating sets, port-numbering model, deterministic algorithms • Best possible approximation factors, exactly matching upper and lower bounds • Open problem: • Can you do better in time f ( ∆ ) if you have unique identifiers instead of mere port numbering? 31
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